The present invention relates to platinum group metal alloy electrocatalysts and to fuel cell electrodes utilizing such electrocatalysts. More specifically, the present invention concerns platinum-gallium alloy electrocatalysts and acid electrolyte fuel cell cathodes utilizing the same.
Generally speaking, a fuel cell is an electrochemical device for obtaining direct conversion of a fuel, such as hydrogen or a hydrocarbon, and an oxidant, such as oxygen, to a low voltage DC current. Typically, such cells comprise fuel electrodes (anodes), oxidant electrodes (cathodes), an electrolyte between the electrodes, and means for separately introducing fuel and oxidant streams to, respectively, the anodes and cathodes.
In operation, the fuel is oxidized at the anode in contact with an electrocatalyst and in the presence of the electrolyte, and results in liberation of electrons. Oxygen or an oxygen-containing gas such as air is fed to the cathode, where it is reduced at the electrocatalytic surface in the presence of the electrolyte, with corresponding consumption of electrons which have been transferred from the anode via an external circuit. The electron flow from the anode to the cathode constitutes a direct current electrical potential which can be usefully applied as such to perform tasks, or converted to alternating current.
Generally speaking, the "platinum group metals" of Group VIII of the Periodic Table, i.e., platinum, palladium, rhodium, ruthenium, iridium, and osmium, or combinations of two or more thereof, have found use as oxygen reduction catalysts and may also be useful as fuel (hydrocarbon or hydrogen) oxidation catalysts. Alloys of such platinum group metals, such as alloys with metals of Group IB of the Periodic Table or other metals, are known for such use. The metal catatyst is usually a supported metal catalyst, that is, the metal is supported on a carrier material such as conductive carbon black powder.
A variety of materials for the support member on which the catalyst is deposited to form the cell electrodes have been utilized with varying degrees of success. Support members made of metal, such as nickel, for instance, can be used to make a metal electrode by securing the catalyst particles onto the support member. Such metal electrodes are permeable to gaseous fuel, such as hydrogen, and are relatively easy to fabricate. They are, however, susceptible to corrosive attack by strong acid electrolytes, such as phosphoric acid, causing shortened life, substantial maintenance expense and attendant variations in electrical output. Carbon electrodes having platinum group metal catalysts applied thereon have been used with some success.
When platinum group metal-containing carbon-supported catalyst compositions are employed as the electrocatalyst of the cathodes of a phosphoric acid fuel cell system, (See J. Electrochem Soc. 127 1219, 1980), the surface area of the metal catalyst tends to progressively decrease. This shortens the effcient operating life span of the electrode and necessitates the added expense and inconvenience of built-in over-capacity to accommodate the anticipated decline. The surface area reduction problem is sometimes referred to as "sintering", and is believed to be caused by the migration of small platinum group metal (e.g., platinum) crystallites across the cathode surface, which causes the crystallites to form larger crystallite masses. The resulting loss of active metal surface area is correlated to progressive loss in cell output and overall efficiency.
Previous attempts at improving electrode catalysts for use in fuel cells in general have included the preparation of an alloy of a platinum group metal with various base metals such as vanadium, tungsten, aluminum, titanium, silicon, cerium, strontium, or the like. See U.S. Pat. Nos. 4,202,934 and 4,186,110. Other attempts have included the deposition of a plurality of metal salts on a support and reduction of the salts to the metals; see U.S. Pat. No. 3,340,097.